(1) Field of the Invention
The present invention relates to a liquid coating composition for the formation of a transparent conductive film and a process for forming a transparent conductive film on a substrate by using this liquid coating composition.
(2) Description of the Prior Art
Transparent conductive films have been broadly used for image pick-up tubes, de-icing materials for automobiles and airplanes, electric screens, electrodes of fluorescent indicator tubes, liquid crystal display devices and the like. These transparent conductive materials are required to have high transparency, good electrical conductivity and high mechanical strength. As materials providing transparent films meeting such requirements, there have been used materials composed mainly of metal oxides such as tin oxide and indium oxide. Indium oxide type materials are ordinarily inferior to tin oxide type materials with respect to thermal stability. However, since films formed from indium oxide type materials can easily be etched by a hydrochloric acid solution, these materials are frequently used for formation of transparent electrodes on which a delicate pattern should be formed, such as those of liquid crystal display devices. As glass substrates for formation of liquid crystal display devices, sodium glass sheets are mainly used for the purpose of reducing manufacturing costs. Since the melting point of sodium glass is low, it is necessary to adopt a temperature as low as possible for the coating operation, and, simultaneously, it is necessary to impart good electrical conductivity and high mechanical strength to the resulting films.
A vacuum evaporation coating process has heretofore been mainly adopted for formation of conductive films of indium oxide type materials. Although this process is satisfactory with respect to the quality of the resulting films, since the melting point of starting indium oxide is high, high-temperature heating is indispensable and the equipment is inevitably expensive and since the process is carried out batchwise, mass production is very difficult. Accordingly, attempts have been made to develop processes in which the coating operation can be facilitated and the foregoing disadvantages can be eliminated. For example, Japanese Patent Publication No. 3282/56 proposes a process comprising coating a substrate with a varnish comprising basic indium trifluoroacetate and heating the coated substrate at a temperature higher than 600.degree. C. to form a conductive film of the indium oxyfluoride (InOF) type. In this process, no good conductivity can be obtained unless the heat treatment temperature is elevated to about 700.degree. C. Accordingly, if a substrate having a low melting point, such as a sodium glass substrate, is employed, no good results can be obtained. Further, Japanese Patent Application Laid-Open Specification No. 37763/77 discloses a process comprising coating a substrate with a coating solution formed by dissolving an inorganic salt of indium in an organic solvent and heating the coated substrate. This process, however, is disadvantageous in that the used indium salt is precipitated in the formed film and the film becomes opaque. Moreover, Japanese Patent Application Laid-Open Specification No. 75991/76 discloses a process for forming conductive films free of such opaqueness, which comprises spray-coating a substrate with a liquid coating composition formed by adding hydrogen fluoride to a solution of indium chloride in water or an alcohol and heating the coated substrate at 420.degree. C. to form an indium fluoride type film. Still further, Japanese Patent Application Laid-Open Specification No. 1497/77 teaches a process for forming an indium oxide type conductive film, which comprises coating a substrate with an organic solvent solution of indium naphthenate and diethyldiethoxyindium and exposing the coated substrate to a high temperature. In this process using organic indium compounds, a disadvantage that the film is rendered opaque by precipitation of crystals is eliminated, but at the sintering step, decomposition of the organic compounds occurs, and the resulting film is rendered porous and carbon is readily left in the film. As a result, the conductivity is reduced the mechanical strength is lowered and there is a further disadvantage in that the film is readily scratched or damaged.